Does the order of definitions matter in languages?

[Ingwie Phoenix]

Hello!

After having tried and waited for several other parsing libraries to provide a HTML and CSS parser, and having gotten nothing out of it other than some knowledge, I have given up and want to write my own parsers using MPC.

Now, I am far from an expert in parsing and really just getting started - and, out of frustration, no less. ;)

So I wanted to start with something very, very simple: In CSS, seeing .foo > .bar is not uncommon, so I wanted to implement a basic version of that in mpc to learn how to start with the very basics:

#include "mpc.h"

int main() {
  char* input = "a>b";
  mpc_parser_t* ident = mpc_new("ident");
  mpc_parser_t* desc = mpc_new("desc");
  mpc_parser_t* sel = mpc_new("sel");
  mpca_lang(MPCA_LANG_DEFAULT,
    "ident = /[0-9a-zA-Z]+/ ;"
    "desc = '>' ;"
    "sel = <ident> | (<ident> <desc> <ident>)+ ;",
    ident, desc, sel, NULL
  );
  mpc_result_t r;
  if (mpc_parse("input", input, sel, &r)) {
    mpc_ast_print(r.output);
    mpc_ast_delete(r.output);
  } else {
    mpc_err_print(r.error);
    mpc_err_delete(r.error);
  }
}

This, however, tells me:

./foo
input: error: Parser Undefined!

My goal was to parse either "a" or "a>b" for now, and later add subsequent further depths with the > operator, like a>b>c>d.... What is the problem here, exactly? Did I order the rules incorrectly or something?

Thanks and kind regards!

(PS. my idea is to parse basic CSS to extract properties and feed it to LVGL widget properties and use HTML as a "commonly known way" to describe a UI. Think Electron, but much, much more minimal...intended for tiny tools at best.)

[Sam Stuewe]

I'm not sure if this will solve your issue completely, but you appear to be using = in the text of mpca_lang() when you should be using :.

[Carlos Rueda]

Just exploring MPC (which looks great so far!)

... and let me chime in with the mentioned fix as well as one more (switch order of the or options for sel):

int main() {
    const char* input = "a>b";
    mpc_result_t r;
    mpc_parser_t* ident = mpc_new("ident");
    mpc_parser_t* desc = mpc_new("desc");
    mpc_parser_t* sel = mpc_new("sel");
    mpca_lang(MPCA_LANG_DEFAULT,
              "ident : /[0-9a-zA-Z]+/ ;"
              "desc  : '>' ;"
              "sel   : (<ident> <desc> <ident>)+ | <ident> ;",
              ident, desc, sel, NULL
    );
    if (mpc_parse("input", input, sel, &r)) {
        mpc_ast_print(r.output);
        mpc_ast_delete(r.output);
    } else {
        mpc_err_print(r.error);
        mpc_err_delete(r.error);
    }
}

Now it runs ok:

>
  ident|regex:1:1 'a'
  desc|char:1:2 '>'
  ident|regex:1:3 'b'